Orthopaedic surgery includes the replacement of damaged or worn joints with prostheses or implants. It is desirable to perform some pre-operative planning in order to select a prosthesis which is an appropriate size for the patient. Typically, this is done by using templates representing a two-dimensional projection of a three-dimensional prosthesis. The templates are in the form of pre-printed acetate sheets which are overlaid on X-ray images of the joint which is to be replaced. This technique allows a surgeon to choose a suitable prosthesis from those available, and also to plan surgical cut lines and determine the required orientation of the prosthesis. The X-ray image with its overlaid template is referred to by the surgeon during the operation.
However, there are a number of disadvantages associated with this templating technique. In general, it only provides an approximate guide to prosthesis size because X-ray images have a range of magnifications, which for any given image may not correspond with magnifications assumed in available templates. In any case, the exact magnification may not be known. Accuracy may also be compromised by the orientation of bones shown in the image; any bone lying in a plane non-parallel to the image plane will appear foreshortened so that its length and position will be difficult to determine.
Any inaccuracies in the planning process can lead to the selection of an unsuitable prosthesis. This may result in increased length of the subsequent operation, as the surgeon may require an alternative prosthesis to be obtained or may have to make additional or further incisions beyond those planned. In the longer term, a poorly selected prosthesis is more likely to fail early or be subject to other post-operative complications, and require substitution at a later date.
All of these factors tend to increase costs by requiring extra hospital staff time and patient care, and also cause increased pain and inconvenience to the patient.
Consequently, it is desired to provide an improved orthopaedic surgery planning method.